● Finding of input impedance and output impedance ●Impedance calculated for audioamplifier, loudspeaker, or microphoneThe usual common question is: How to measure impedance?The question of the input impedance of a microphone or the output impedance of a loudspeaker is nonsense.
There is only the output impedance of a microphone and the input impedance of a loudspeaker.
What is the load resistance of an audio amplifier? That is the loudspeaker.

A resistance is a DC resistance, which can be measured with an ohmmeter. If there is a
capacitor in the signal path we measure nothing. With a voice coil of a speaker we can
measure a DC resistance. With a digital multimeter DMM resistances can be measured easily,
but we cannot measure input impedances and output impedances. The capacitance and the
resistance build as frequency-dependent form a complex resistance, the so-called impedance
Z. The nominal impedance is in electrical engineering and electro-acoustic (audio), the
frequency-dependent impedance at the input and / or at the output of an electrical device,
which is specified in the middle frequency range at 1 kHz of a technical data sheet.
In electrical engineering and acoustics alternating quantities are always described with its
effective value (RMS).

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When the voltage V2 is equal to half of V1, then the
measured resistance
value Rload (that is Rtest) is equal to the output impedance Zsource.

Zsource = output impedance = source impedance = internal impedance.

The output impedance of a device can simply be determined. We use a
load resistance Rload, to load the signal source impedance Zsource. The
output voltage is open initially without load as open-circuit voltage V1
(Switch is open, that means Rload is infinity) and then measured as V2
under load with Rload at point IN (Switch is closed). Then the found
values V1, Rload and V2 are entered to calculate the output impedance.
The load resistance Rload should not be too small, because the output is
too heavily burdened and should not be too large, as this will change the
voltage very little and leads to measurement errors. For output
impedance of a normal power amplifier to operate a speaker a Rload
resistance of about 10 ohms is favorable.
For other line-level RLoad a resistance of 2 kilo ohms is useful.

Internal resistance of a power amplifier

"Measuring the output impedance by means of a burden": Suppose there
is a 100 watt amplifier. Then the output voltage at half power is P = 50 W
= V2 / R. Loudspeaker impedance = 8 ohms. V = √(P × R) = √ (50 × 8)
= 20 volts. (You can also use 10 V.) Give a sine voltage of 1 kHz to the
amplifier input, until we get 20 volts at the output. Now we apply the "90%
method", that is when we put an output resistance R, until there appear
90% of the open circuit voltage, in this case 18 volts. The internal
resistance is then calculated with the 90% method:

The 90% methodRinternal = R / 9

At the output fix an oscilloscope, because the wave form should not show
any distortion.
For example, if R is measured 1 Ohm, then Rinternal = 0.11 Ohm.

Input Impedance Measurement and Calculator

Generator signal V1

volts

Series resistance Rtest

ohms

Signal after resistor V2

volts

↓

Input impedance Zload

ohms

Input impedance

Voltage measurement at the points IN or at OUT:V1 = Generator signal voltage (at Rs = 0 Ω, that is without series resistor Rs)Rs = Series resistance (Rtest is resistor to measure Ω value)V2 = Voltage with series resistor Rs = resistance RtestZload = The input impedance can be calculated

When the voltage V2 is equal to half of V1, then the measured
resistance value
Rs (Rtest) is equal to the input impedance Zload.

The input and output impedance of a four-terminal network can be
determined by measuring the alternating current strength in amperes and
the AC voltage in volts. The measurement of input impedance typically
occurs as follows: The voltage is measured across the input terminals IN.
Then, the current in the circuit is done by the device in series with the
signal generator. For circuits with high input impedance the current is
very small and difficult to measure. R = U / I. Therefore, we choose for
the measurement of high-impedance circuits, a better method. It puts a
series resistor Rs in the input circuit. First, we measure the input of the
device
at point IN with V1, the AC voltage, if the resistor Rs = 0 Ohm.
Then we measure the RS series resistor, the voltage V2. Then these
found valuesV1, Rs and V2 is entered in the above calculator to find the
input impedance to be calculated. Search for a suitable measuring
resistance value Rs. For typical audio equipment that will be about 10 to
100 kilo-ohms.You can use the digital voltmeter instead at the measuring point IN and
at point OUT to measure because the amplifier delivers an output
voltage that is proportional to the voltage at its input.

The impact of input impedance and output impedance of
studio gear for bridging in audio engineering − Zsource <<Zload

Fortunately, there are no amplifiers with an output impedance of
4-ohm or 8-ohm which have to fit to speakers with these values.
We have no impedance matching (power matching), we use
impedance bridging (voltage bridging), whereby the power
amplifier often has an output impedance of only one hundredth of
the speaker's input impedance.
At power amplifiers for musicians usually we can read at the
output plugs: 4 ohms to 8 ohms − to tell the user that a 4-ohm
speaker or an 8-ohm speaker has to be used and not to give the
"correct" output impedance value, which is around 0.1 ohms.
This is often not known by users.

The word "power amplifier" is a misnomer - especially in audio engineering.
Voltage and current can be amplified. The strange term "power amplifier" has
become understood to mean an amplifier that is intended to drive a load such
as a loudspeaker.
We call the product of current and voltage gain "power amplification".